The only nonterminal symbol in this grammar is <expression>,
which is also the start symbol.
The terminal symbols are

{ +, -, *, /, (, ), number } .

(We will interpret "number" to represent any valid number.)

The first rule (or production) states that an <expression>
can be rewritten as (or replaced by) a number.
In other words, a number is a valid expression.

The second rule says that an <expression> enclosed in
parentheses is also an <expression>.
Note that this rule defines an expression in terms of expressions,
an example of the use of recursion in the definition of context-free
grammars.

The remaining rules say that the sum, difference, product,
or division of two <expression>s is also an expression.